Respiration rate is a vital sign that is monitored either intermittently or continuously in a variety of situations, including but not limited to during and after surgical intervention. Continuous monitoring apparatus can be placed in contact with the body (abdominal belts or air flow on trachea or mask) or non-contact (e.g. pressure sensors under mattress, or a Doppler sensor). Continuous monitoring of respiration beyond the respiratory rate (such as breathing patterns, breathing depth, minute ventilation and apnea) is achieved nowadays using contact and invasive sensors.